Lithium secondary battery has been commercially available for more than twenty years and remarkably developed its capacity and performance. In recent years, the lithium secondary battery has also been increasingly used as an in-car power supply in addition to as a power supply of an information-communication device, and thus is expected to provide higher capacity and higher output.
However, there are some problems: a graphite negative electrode comprising a graphite system as an active material is used for a commercially available lithium secondary battery and thus an electrolytic solution is limited to an ethylene carbonate (EC) system; and the electrode and the electrolytic solution tend to react when the battery's temperature reaches more than 45° C., causing the lithium secondary battery to be severely deteriorated. On the other hand, when the battery's temperature is 0° C. or less, Li dendrite is likely to generate at the time of charge. Accordingly, it is necessary to prevent a short circuit using a microporous separator with pore size of 100 nm or less, and thus a separator such as a woven fabric and a nonwoven fabric has not been preferably used. Furthermore, a carbon-based negative electrode has higher conductivity in a direction of electrode plane, and thus a large current flow in a short-circuited part in an internal short circuit test, such as nail penetration, rapidly produces heat, resulting in thermal runaway of a battery.
In order to break limits of conventional lithium secondary batteries and expand the industrial field, any change is essential and thus research and development of any negative electrode material such as Sn system and Si system has been actively performed.
The inventors have reported that Sn—Sb based sulfide glass functions as a negative electrode material of a lithium secondary battery or a sodium secondary battery and stably works even under the environment of −20 to 60° C. It has been found that when in particular used for the lithium secondary battery, Sn—Sb based sulfide glass can be combined with Si to obtain stable cycle life in a capacity of 1000-2000 mAh/g (nonpatent literatures 1-3).
However, Sn—Sb based sulfide glass requires special facilities since it is obtained by fusion in hot sulfur gas atmosphere at the temperature of about 1000° C. Furthermore, the Sn—Sb based sulfide glass is combined with Si by further mechanical milling process, which is complex and accordingly raises the production cost.